Letharia

Taxonomy

Historically only two species were recognised in Letharia: L. vulpina and L. columbiana. The traditional distinction between the two species was treated as a classic lichen species pair, with sorediate thalli assigned to L. vulpina and fertile thalli with abundant ascomata assigned to L. columbiana. Altermann and colleagues noted, however, that this separation was imperfect, because some L. columbiana specimens also produce isidia, while typically sorediate L. vulpina can occasionally bear apothecia. A 2016 multi-locus molecular study that sequenced three fungal and two algal markers from 302 thalli retrieved at least six well-supported, reproductively independent lineages—two within the traditional L. vulpina concept and four within L. columbiana sensu lato. One of the former was formally described as Letharia lupina. To stabilise usage of the name L. columbiana, Altermann and colleages designated a freshly sequenced specimen from the Walla Walla River valley (Oregon) as an epitype, because the 1833 holotype is a degraded, mixed gathering dominated by L. vulpina and riddled with contaminants. Three further lineages—L. gracilis, L. barbata and L. rugosa—were judged likely to merit species rank once additional diagnostic data become available. Each fungal species partners exclusively with a distinct photobiont clade inside Trebouxia jamesii sensu lato, showing that Letharia represents several discrete symbioses rather than a single variable species complex. An earlier multilocus study based on 432 specimens broadly recovered the same six candidate lineages proposed in earlier molecular work, but found that the strongest and most consistent separation was between the sorediate groups later treated as L. lupina and L. vulpina, while relationships among the apotheciate lineages remained less certain.

The same work shows that western North America is the centre of diversity for the group: L. lupina alone accounted for 88% of nearly 300 North American collections and ranges from valley bottoms to the alpine tree line, extending east of the Continental Divide into Alberta and the northern Rocky Mountains. By contrast, L. vulpina is largely confined to lower, drier conifer forests and is much rarer on the continent, though it remains the prevalent Letharia in Europe. Traditional morphological characters such as branch density, branch colour and isidial abundance were found to vary widely within species, rendering them unreliable for routine identification; instead, fixed nucleotide differences in the DNA sequence, or PCR-RFLP assays, allow rapid discrimination between L. vulpina and L. lupina.